Evidence in the last decade implicates cerebral microvasculature abnormalities in the genesis of AD neuropathology. Additional literature shows that the EphB4/ephrinB2 system is an important regulator of the development and function of the vascular system. Binding of the extracellular sequence of EphB4 receptor to its transmembrane ligand ephrinB2 protein on the surface of endothelial cells of blood vessels, stimulates angiogenesis and growth of new vessels from existing vasculature. Furthermore, in vitro assays show that treatment of ephrinB2expressing endothelial cells with the extracellular domain of EphB4 stimulates cell sprouting and tube formation, processes considered crucial initial steps in angiogenesis, while transgenic mouse experiments indicate that the intracellular (cytoplasmic) domain of ephrinB2 protein is necessary for ephrinB2dependent angiogenesis. We found that EphB4 stimulates the metalloproteinase (MP) and PS/?secretase processing of ephrinB2 producing cytosolic peptide ephrinB2/CTF2 and that the EphB4induced sprouting and tube formation of endothelial cells depends on ?secretase activity. These observations raise the possibility that the endothelial EphB4/ephrinB2 system regulates angiogenesis through PS/?secretase. In support of this hypothesis, we obtained data that peptide ephrinB2/CTF2 stimulates sprouting of endothelial cells in vitro. Recent literature shows that a crucial step in angiogenic factorinduced angiogenesis is formation of complexes between Raf1/ Rok? and Vascular Endothelial cadherin (VEcadherin) and we made the novel observation (preliminary data) that treatment of endothelial cell cultures with EphB4 increases these angiogenic complexes. Together, our observations suggest that PS1/?secretase may affect angiogenesis by regulating processing of transmembrane protein ephrinB2, a critical step in EphB4induced angiogenesis. Here we propose to explore the mechanisms via which the EphB4/ephrinB2 and PS1/?secretase systems interact to promote endothelial cell sprouting and angiogenesis and to examine whether any of these mechanisms are altered in Alzheimer disease (AD) brains. Furthermore, we and others reported that PS1 familial AD (FAD) mutations may affect the epsilon (?) cleavage of PS1/?secretase substrates thus decreasing production of CTF2 peptides including ephrinB2/CTF2 (see Significance). Thus, we will ask whether PS1 FAD mutants alter the EphB4/ephrinB2dependent angiogenesis.